How to Grow Bacterium in Petri Dish

Written by patricia voldberg

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The safest way to grow bacteria is in a petri dish. (Bacteria Colonies image by ggw from Fotolia.com)

Bacteria are everywhere. They are on every surface. Given optimum growing conditions, a bacterium can grow rapidly. The main reason they don't cover the whole earth is that conditions for them to multiply are rarely perfect. Most bacteria found in the environment are not harmful. However, huge colonies of bacteria can become dangerous. Bacteria are not usually visible to the human eye. The best place to grow bacteria into colonies is in special containers called petri dishes.

Skill level:

Moderate

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Things you need

Prepared petri dishes containing agar

Bacteria collected from difference sources

Masking tape

Marking pen

Sterile swabs

Plastic bag

Paper bag

Bleach

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Instructions

1

Purchase two sterile petri dishes full of the nutrient agar online or from a science supply store. Bacteria need a food source, and agar is a good growth medium for culturing bacteria. Agar comes from red algae and is a type of carbohydrate, which is ideal for growing microorganisms.

2

Store petri dishes upside-down to keep condensation from forming on the lid. Condensation can drip into the medium and disrupt the bacteria growth. Before using the petri dish, warm it up to room temperature for about an hour before obtaining any samples.

3

Write the letter "A" on a piece of masking tape with a marking pen and place it on the underside of one petri dish. Label the other petri dish with the letter "B." The "B" dish is the control dish in the experiment, and an identical test where the independent variable is left unchanged to provide a comparison.

4

Turn the petri dishes right side up, and with the marking pen, divide the tops of the petri dishes into four equal parts. Number the sections 1, 2, 3 and 4.

5

Collect bacteria from the locations you have chosen. Use one swab for each area. To get a good sample, wet the end of a cotton swab with water. Swipe the swab's tip all over the surface of the object.

6

Raise the lid of petri dish A just enough to get the swab inside. Make a large X across the nutrient agar with the cotton swab. Do not tear into the agar. Close the petri dish lid, immediately. Do not touch the agar with your hand. Do not open the petri dish B.

7

Replace the cover on the dish, tape it closed, and label each dish with the source of the bacteria. Store the dish upside down. Place the petri dish inside a plastic bag and seal it. View growing bacteria through the clear plastic bag.

8

Place the petri dishes in a warm, dark place for 4 to 6 days. Some cultures grow better in the dark, so if necessary place the sealed dishes in a paper bag.

9

Observe each dish daily without opening them. After a few days, the first quadrant in the A dish will show the heaviest bacteria growth. Depending on the amount of bacteria on the swab, the fourth quadrant will show none or very little growth. It is easier to distinguish the types of bacteria in low-growth or less cluttered areas.

10

Kill the bacteria in the petri dish before disposing of dishes in the trash. Pour household bleach over the colonies while holding dish over garbage. Be sure to wash hands well after disposing petri dishes.